Reconstruction of soft tissues using a silicone elastomer bag filled with silicone gel is a common surgical procedure. Such an implant was described by Cronin in U.S. Pat. No. 3,293,663 for reconstruction of the human breast. However, after a short period of time a capsule composed of fibrous scar tissue forms around the implant. It is commonly believed that silicone gel "bleeding" through the bag causes an inflammatory response which results in this capsular formation. Thickening and eventual contracture of the fibrous capsule results in hardening and spherical deformation of the implant and surrounding tissues. The implant becomes painful, aesthetically unacceptable, and can cause erosion of the overlying tissues.
The use of saline filled silicone elastomer bags and double-lumen implants with the outer chamber containing saline, decreases the inflammatory response. However, failure of the silicone elastomer bag, especially along folds, is more common with saline filled implants. This is due to abrasion of the bag against itself, frequent flexing of the material as the patient moves, the low viscosity of the filling material, and the decreased lubricity of the saline compared to silicone gel. Rupture of a saline filled implant allows the tissue cavity to shrink as the saline is absorbed into the surrounding tissues.
U.S. Pat. No. 4,157,085 to Austad discloses hydrophilic polymers such as poly-N-vinylpyrrolidone, carboxymethylcellulose, or polyethylene glycol encapsulated within a membrane permeable to extracellular body fluids under osmotic pressure. The preferred material is a very thin silicone membrane capable of transmitting fluids as well as stretching as the fluid concentration of the enclosed material increases. This device is intended to be used to stretch tissue as the polymer inside the envelope absorbs fluid. When tissue expansion is completed, the device is removed and replaced with a suitable prosthesis. This is necessary since the polymers inside the envelope are water soluble, not crosslinked, and would readily disperse in the body if they should escape from the device if its membrane ruptured or tore.
Polmanteer in U.S. Pat. No. 4,138,382 discusses the use of hydrophilic gels which are copolymers of olefinic hydrolyzable silanes and water soluble vinylic constituents. These gels swell in the presence of water to form a loose crosslinked network using siloxane [.tbd.Si--O--Si.tbd.] as the covalent crosslinking entity. However, this results in a gel which can dissociate in water according to the equilibrium reaction ##STR1## and become soluble. Such gels would slowly be absorbed into the tissue in the event of a rupture or tear in the envelope.
U.S. Pat. No. 4,517,326 to Cordts suggests the use of a polyurethane gel containing an aqueous dispersion for use as an implantable prosthesis. The water level in the gel can only be varied from 25 to 65 percent which limits the softness of the device. Additionally, such a device would contain macroporosity in the form of dispersed water droplets and be susceptible to calcification and tissue ingrowth.
Various medical researchers have evaluated the implantation of poly(hydroxyethyl methacrylate (pHEMA)) as a breast tissue replacement. Depending on the water content, cross-linking agent, monomer content, and pore structure as well as other variables within these implants, a broad range of tissue interaction and encapsulation, small and giant cell growth, vascularization and calcification has been demonstrated. In general, pHEMA hydrogels with high water contents exhibit poorer mechanical properties, tend to calcify, are difficult to shape, and are readily damaged during implantation. This is believed to be due to their macroporous structure at high water contents. Lower water content pHEMA hydrogels which are homogeneous or have only microporosity do not exhibit calcification. However, they are generally stiffer and less malleable and have a greater tendency to incite a fibrous capsule.
The success of post-mastectomy reconstruction, as well as other procedures involving implants of a soft tissue prosthesis, would be greatly enhanced by the use of a prosthesis which is soft and malleable, does not calcify or incite severe fibrous encapsulation, and is resistant to leakage in the event of envelope rupture or tear. The present invention describes such a soft tissue prosthesis containing a less reactive, homogeneous, and more biocompatable filling along with construction designs which provide long term stability even in the event of envelope tear or rupture.